My estimate for end of the year market share (weekly sales are here: http://www.vgchartz.com/forum/thread.php?id=45704&start=50).
Wii: 45.52M - 48.5%
Ps3: 21.67M - 23.1%
X360: 26.64M - 28.3%
The gap will then be 2.79M
I see the Wii selling 2M-2.2M/month in Q1 next year, which is ~ 484K weekly. I see the HD consoles dipping as most consoles would, down to ~ 180K for Ps3 and 150K for X360. 330K total.
So I see it taking 18 weeks or so (into 2009) for Wii to achieve 50% market share, and it happening roughly at the start of May.
Note: This is not a purely statistical post, and it has a few personal estimates that are not totally proof backed. No cakes were hurt during the making of this Barack Obama ad.
http://www.vgchartz.com/games/userreviewdisp.php?id=261
That is VGChartz LONGEST review. And it's NOT Cute Kitten DS